Method and apparatus for forming expitaxial layers

ABSTRACT

The present invention provides a method of depositing epitaxial layers based on Group IV elements on a silicon substrate by Chemical Vapor Deposition, wherein nitrogen or one of the noble gases is used as a carrier gas, and the invention further provides a Chemical Vapor Deposition apparatus ( 10 ) comprising a chamber ( 12 ) having a gas input port ( 14 ) and a gas output port ( 16 ), and means ( 18 ) for mounting a silicon substrate within the chamber ( 12 ), said apparatus further including a gas source connected to the input port and arranged to provide nitrogen or a noble gas as a carrier gas.

The present invention relates to a method of manufacturing asemiconductor device comprising the step of forming an epitaxial layeron a silicon substrate in particular by means of Chemical VaporDeposition (CVD), and to an apparatus therefor.

In order to extend the material variety for devices employing siliconmicroelectronic technology, materials such as Si_(1-x-y)Ge_(x)C_(y) (andSi_(1-y)C_(y)) crystals epitaxially grown on Si substrates havegenerated much interest.

Silicon: Carbon (Si_(1-y)C_(y)) is a new material discussed in, forexample, J. P. Liu and H. J. Osten, “Substitutional carbon incorporationduring Si1-x-yGexCy growth on Si(100) by molecular-beam epitaxy:Dependence on germanium and carbon” Applied Physics Letters, Vol. 76,No. 24, (2000), P. 3546-48 and H. J. Osten, “MBE growth and propertiesof supersaturated, carbon-containing silicon/germanium alloys onSi(100)”, Thin Solid Films, Vol. 367, (2000), P. 101-111, and it has thesame diamond structure as silicon. Typically, the C concentration isbetween 0 and 5%, i.e. well beyond the solubility limit of C in Si. InSi_(1-y)C_(y), all carbon atoms should be substitutional, and take theplace of a silicon atom in the silicon structure. Therefore a particularchallenge for the elaboration of this material is to introducesubstitutional carbon into silicon. As noted, the equilibrium solidsolubility of C atoms in Si and Ge is extremely small, therefore, it canbe desirous to grow high quality Si_(1-x-y)Ge_(x)C_(y) crystals with asubstitutional C concentration up to a few at %. The fraction ofsubstitutional carbon increases at low temperature. If the carbon atomsare not in a lattice position, that is if they are not substitutional,they can lead to a variety of defects such as, but not limited to, SiCprecipitates. Such defects are not suitable for semiconductorapplications and epitaxial growth.

It has been shown that Si_(1-x-y)Ge_(x)C_(y) and Si_(1-y)C_(y) epitaxiallayers can be grown on a Si(001) substrate using various techniques suchas Molecular Beam Epitaxy (MBE), Rapid Thermal Chemical Vapor Deposition(RT-CVD), Low Pressure Chemical Vapor Deposition (LP-CVD) and Ultra HighVacuum Chemical Vapor Deposition (UHV-CVD). The latter being known fromB. Tillack, B. Heinemann, D. Knoll “Atomic layer doping ofSiGe—fundamentals and device applications”, Thin Solid Films, Vol. 369,(2000), p. 189-194; Y. Kanzawa, K. Nozawa, T. Saitoh and M. Kubo,“Dependence of substitutional C incorporation on Ge content forSi1-x-yGexCy crystals grown by ultrahigh vacuum chemical vapordeposition”, Applied Physics Letters, Vol. 77, No. 24, (2000), P.3962-64 and S. John. E. J. Quinomes, B. Ferguson, S. K. Ray, B.Anantharan, S. Middlebrooks, C. B. Mullins, J. Ekerdt, J. Rawlings andS. K. Banerjee, “Properties of Si1-x-yGexCy epitaxial films grown byUltrahigh Vacuum Chemical Vapor Deposition”, Journal of TheElectrochemical Society, Vol. 146, No. 12 (1999), P. 4611-4618. Oneparticularly important factor is the relationship between substitutionaland interstitial carbon incorporation, which has an impact on theelectrical and optical properties of the layer. It is thought that thecarbon substitutionality, i.e. the fraction of substitutionalincorporated carbon atoms, and the crystal quality, are stronglyinfluenced by growth conditions.

At low temperatures, that is generally below 600° C., the growth rate ofepitaxial layers is controlled by the presence of hydrogen atoms on thesurface, which results from the decomposition of molecule gases such asSiH₄, Si₂H₆, SiH₂Cl₂, GeH₄, H₂ during the deposition process, and whosedesorption process is not immediate. The equilibrium hydrogen coverageon silicon as a function of the hydrogen pressure and temperature can bemodeled by the Langmuir model adsorption as discussed in P. V. Schwartzand J. C. Sturm, “Oxygen incorporation during low temperature chemicalvapor deposition growth of epitaxial silicon films”, Journal of theElectrochemical Society, Vol. 141, No. 5, (1994), P. 1284-1290. For thegrowth of epitaxial layers, however, hydrogen is very much dominant asthe current carrier gas of choice.

It should be appreciated, and is discussed below, that hydrogen plays acrucial role in epitaxy. All CVD processes use hydrogen as the carriergas for reasons such as those mentioned above.

It is noted that when Si is grown by CVD from a gas precursor such asSiH₄ at low temperature, hydrogen passivates the surface, rendering itinert to contaminants that would prevent epitaxy. Furthermore, it isbelieved that hydrogen may be beneficial for epitaxy by contributing tointerface abruptness for hetero-layers and by reducing surfaceabsorption and diffusion during growth. Analogous behavior has also beenfound when a variety of dopants are absorbed on the growth front. Thisalso explains why as known from E. Finkman, F. Meyer and M. Mamor,“Short-range order and strain in SiGeC alloys probed by phonons”,Journal of Applied Physics, Vol. 89, No. 5 (2001), p. 2580-2587,attempts are being made to employ hydrogen in the Molecular Beam Epitaxy(MBE) technique where no carrier gas is in fact required.

Further, the effect of germane on hydrogen desorption is known from M.Liehr, C. M. Greenlief, S. R. Kasi and M. Offenberg, “Kinetics ofsilicon epitaxy using SiH4 in a rapid thermal chemical vapor depositionreactor”, Applied Physics Letters, Vol. 56, No. 7, (1990), P. 629-631.The results discussed are widely used in the SiGe growth models andreport that germane has an enormous catalytic effect on the growth rateof silicon at low temperature. This catalytic effect increases thehydrogen desorption from germanium sites on the growth surface, allowingincreased adsorption of the growth species and hence an enhanced growthrate. This document concludes that in low-temperature vapor phaseepitaxy, a small addition of germane can increase the effective growthrate of silicon by two orders of magnitude. Although the exact mechanismresponsible for the catalysis is not known, this effect is veryfavorable because it enables growth rates to be obtained for theGe_(x)Si_(1-x) films (100 Å/min) at low temperatures without the use oflasers, plasmas, or other exotic growth enhancement techniques.

It is also known that hydrogen works as a surfactant in SiGe epitaxy,suppressing three-dimensional growth which can lead to roughness orisland formation of SiGe as discussed in J. Vizoso, F. Martin, J. Suneand M. Nafria, “Hydrogen desorption in SiGe films: A diffusion limitedprocess”, Applied Physics Letters, Vol. 70, No. 24, (1997), p. 3287-89.

Thus, for good reason hydrogen has been considered the carrier gas ofchoice and is, in general, an important gas for use in the epitaxialprocess.

WO-A-01/14619 discloses the epitaxial growth of silicon carbide and/orsilicon germanium carbide, and employs nitrogen as a carrier gas.However, the epitaxial reactor disclosed in this document is arranged tooperate at ultra high temperatures, i.e. in the range of 1100-1400° C.

The present invention seeks to provide a method of forming epitaxiallayers on a silicon substrate by means of CVD, which method also offersadvantages over known such methods.

The invention also seeks to provide such a Chemical Vapor Depositionapparatus.

According to one aspect of the present invention there is provided amethod of manufacturing a semiconductor device comprising the step ofdepositing an epitaxial layer based on Group IV elements on a siliconsubstrate by Chemical Vapor Deposition, and comprising employingnitrogen or one of the noble gases as a carrier gas.

In particular, such carrier gases advantageously serve to control thequality of the epi-layers and the carbon incorporation.

In particular, through using nitrogen as a carrier gas it has becomepossible to obtain smooth epitaxial silicon layers at higher growthrates and lower growth temperatures as compared with known Si epitaxyprocesses employing hydrogen as a carrier gas.

The features of some of the claims confirm the particular Group IVelements that can form part of an epitaxial system according to thepresent invention.

The features of some of the claims advantageously further improve thedeposition rate and can serve to limit the cost of production.

Through the use of nitrogen as a carrier gas for low temperature CVDepitaxy of Silicon, Silicon: Carbon, Silicon Germanium and SiliconGermanium Carbon, it has become possible to increase the process windowsconsiderably beyond the limits which are inherent in processes employinghydrogen as the carrier gas. The low temperature epitaxy process withnitrogen as the carrier gas has a different growth mechanism compared tothe low temperature epitaxy process with hydrogen as the carrier gas.This leads to higher quality materials and improved carbonincorporation.

The invention can be employed in particular for depositing doped orundoped Si_(1-x-y)Ge_(x)C_(y) and Si_(1-y)C_(y) epitaxial layers on Sisubstrates at high growth rates, high carbon substitutionality, and lowcost of production. The combination of low growth temperature and highgrowth rates is also attractive for applications requiring low thermalbudget such as Si cap layers on top of a strained SiGe layer with a highGe concentration.

The invention is described further hereinafter, by way of example only,with reference to the accompanying drawings in which:

FIG. 1 is a graphical illustration of the temperature dependence of thegrowth rate, with nitrogen being used as the carrier gas instead ofhydrogen;

FIG. 2 is a graphic representation of the thickness and the germaniumconcentration uniformity over the wafer;

FIG. 3 is a further graphic representation illustrating the benefits ofemploying nitrogen as a carrier gas; and

FIG. 4 is a schematic block diagram illustrating a chemical vapor phasedeposition apparatus for use in accordance with the present invention.

Experiments were conducted with a commercial reduced pressure chemicalvapor deposition reactor, and the following relates to the basic effectson the SiGe growth.

The growth rate of SiGe as a function of temperature using hydrogen andnitrogen as a carrier gas is shown in FIG. 1. The growth conditions fornitrogen were Pressure=40 Torr, N₂=33 slm, SiH₄=5 sccm, GeH₄=150 sccm.For the hydrogen trace the pressure was 40 Torr, H₂=33 slm, SiH₄=20 sccmand GeH₄=150 sccm.

This clearly shows that the growth rate of SiGe using nitrogen as thecarrier gas can be increased by a factor of 10 at a low temperaturecompared to the growth rate of SiGe with hydrogen as the carrier gas. Asimilar effect has also been observed when growing only silicon, inwhich case the growth rate is multiplied by a factor of 6 (at 575° C.and with a silane flow of 400 sccm) when the carrier gas is switchedfrom hydrogen to nitrogen.

The same general growth conditions, i.e. carrier gas flow, growthtemperature and total pressure etc. have been employed in order tohighlight the effect of the carrier gas species on the growth rate. Itis of course noted that in practice this would lead to a very pooruniformity of the SiGe thickness and Ge concentration of the epi-layersover the wafer in the range of 15%, which would be unacceptable forpractical applications.

In the semiconductor industry the uniformity of the thickness and theconstituents concentration of the epitaxial layers is of primeimportance. In this respect the design of the reactor and especially thedirection and magnitude of the gas flow with respect to the position ofthe wafers is a decisive factor. As illustrated in FIG. 2, smoothuniform SiGe(C) epi-layers can be grown with nitrogen as the carrier gasand through adjustment of different growth parameters.

FIG. 3 provides a further illustration of the advantages of employingnitrogen as a carrier gas as compared with hydrogen. FIG. 3 inparticular illustrates the advantages achieved with low temperaturesilicon epitaxy, and for the grown condition of P=40 Torr and N₂=15 slmas illustrated, smooth epitaxial silicon layers can be achieved atrelatively high growth rates and at lower growth temperatures. Higherquality materials, with improved carbon incorporation, can therefore beproduced.

Turning now to FIG. 4, there is illustrated chemical vapor depositionapparatus 10 arranged for use in accordance with the present inventionand comprising an elongate chamber 12 having at one end a gas input port14 and at an opposite end a gas output port 16. The chamber 12 houses apedestal mount 18 upon which is located a silicon substrate 20.

The pressure within the chamber 12 can be controlled by means of themanner in which gas is introduced therein via the input port 14 andextracted therefrom via the output port 16. The input port 14 is incommunication with a gas manifold 22 into which are fed gases from aplurality of sources. In the illustrated example, a first source 24 isillustrated for feeding a first gas by way of the feed line 26 to thegas manifold 22, and a second gas source 28, which in this illustratedexample is arranged for feeding the gas by way of a feed line 30 intothe gas manifold 22.

The total number of gas sources can of course be arranged as requiredwith regard to the epitaxial layer to be formed on the substrate 20.

Tests have been conducted at a growth temperature of 575° C. At thistemperature, the growth rate seems to be mass transport controlled ordiffusion limited controlled, depending on the silane flow, and growthrates around 10 nm/min with a small density of defects have beenobtained. The combination of SiH₄ and SiH₂Cl₂ gases as Si precursorgases may be an alternative depending on the growth temperature. Fortemperatures lower than 550° C., the hydrogen atoms created at thesurface as a by-product of the growth are considered to passivate the Sisurface.

It is known that the low carbon solid solubility in Si makes the carbonincorporation in silicon a critical process.

It has proved possible to grow high quality silicon carbon epitaxiallayers at a growth temperature of 550° C. using silane and nitrogen as acarrier gas.

Smooth and high quality materials can be grown according to the presentinvention. The growth conditions for nitrogen were P=40 Torr, N₂=33 slm,SiH₄=5 sccm, GeH₄=150 sccm; for hydrogen, the conditions were P=20 to 40Torr, H₂=33 slm, SiH₄=20 sccm and GeH₄=150 sccm.

As will therefore be appreciated, through the use of nitrogen as acarrier gas high quality smooth silicon germanium carbon epitaxiallayers were grown with, for example, a germanium concentration in theorder of 20 at. % and a carbon concentration up to 1.3%. The growthconditions for such an example included a deposition temperature of 525°C., a nitrogen flow of 15 slm, a silane (SiH₄) flow of 20 sccm, agermane (GeH₄) flow of 150 sccm and a methyl-silane (SiH₃CH₃) flowbetween 0 and 20 sccm; and the results clearly illustrate the benefit ofthe process of the present invention when employing nitrogen as acarrier gas. Of course the substitutional carbon concentration can beincreased by lowering the growth temperature and by increasing theMano-Methyl-Silane flow.

The invention describes a method of depositing epitaxial layers based onthe Group IV elements Si, Ge, C on a Si substrate in a commercial CVDreactor and, as noted, advantageously employs nitrogen, or a noble gas,as a carrier gas in order to grow high quality epitaxial materials withhigh substitutional carbon content and at a lower cost of productioncompared to currently standard processes using hydrogen as the carriergas.

1. A method of manufacturing a semiconductor device comprising the stepof depositing an epitaxial layer based on Group IV elements on a siliconsubstrate by Chemical Vapor Deposition using source gases, and includingemploying nitrogen as a carrier gas, wherein the epitaxial layercomprises Si_(1-x-y)Ge_(x)C_(y).
 2. The method as claimed in claim 1,which is carried out at a temperature that facilitates a CVD growth rateof an epitaxial layer that is substantially greater than a CVD growthrate of such an epitaxial layer using hydrogen as a carrier gas.
 3. Amethod as claimed in claim 1, which is carried out at a temperature ofless than about 600° C.
 4. A method as claimed in claim 1, which iscarried out at a temperature between 500° C. and 600° C.
 5. A methodclaimed in claim 1, wherein the source gases include SiH₄, GeH₄ andSiH₃CH₃.